Honey, I shrunk the circuit

By Sue Major Holmes

Photography By Randy Montoya

Thursday, January 05, 2017

ULTRAWIDE BANDGAP RESEARCH — Sandia electrical engineer Bob Kaplar heads a project studying ultrawide bandgap semiconductor materials. The project is answering such questions as how materials behave and how to work with them, which are steps toward improving everything from consumer electronics to power grids. (Photo by Randy Montoya)

Sandia researchers have shown it’s possible to make transistors and diodes from advanced semiconductor materials that could perform much better than silicon, the workhorse of the modern electronic world.

The breakthrough work takes a step toward more compact and efficient power electronics, which in turn could improve everything from consumer electronics to electrical grids. Power electronics are vital for electrical systems because they transfer power from its source to the load, or user, by converting voltages, currents, and frequencies. Sandia’s research was published last summer in scientific journals and presented at conferences.

“The goal is to be able to shrink power supplies, power conversion systems,” says Bob Kaplar (1767), who leads a Grand Challenge Laboratory Directed Research and Development project studying ultrawide bandgap (UWBG) semiconductor materials. The project explores ways to grow those materials with fewer defects and create different device designs that exploit the properties of these new materials that have significant advantages over silicon.

The project is laying the scientific groundwork for the new UWBG research area, answering such questions as how the materials behave and how to work with them. It also will aid Sandia’s broader work through developments such as compact power conversion by using better semiconductor devices. “Understanding the science helps lead toward that second goal,” Bob says.

Bandgap is a fundamental materials property that helps determine electrical conductivity and ultimately transistor performance. Wide bandgap (WBG) materials allow devices to operate at higher voltages, frequencies, and temperatures, and are starting to have impact on power conversion systems.

Emerging ultrawide bandgap materials are even more attractive because they could allow further scaling to devices that operate at even higher voltages, frequencies, and temperatures. When made into transistors, the materials have the potential to vastly improve the performance and efficiency of electrical power grids, electric vehicles, computer power supplies, and motors for such things as heating, ventilation, and air conditioning systems. Faster switching also could lead to smaller capacitors and associated circuit components,miniaturizing the entire power system

Highest-bandgap transistor

Sandia researchers demonstrated the highest-bandgap transistor ever, a High Electron Mobility Transistor, and published those results in the July 18 edition of Applied Physics Letters. Sandia published two papers in Electronics Letters analyzing the performance of diodes made from gallium nitride (GaN) and aluminum gallium nitride (AlGaN).

AlGaN DIODE — The image illustrates the breakdown of the AlGaN diode, which is shown in the photo in the center. A diode has to block voltage, and the breakdown value is the maximum voltage that can be blocked. Both curves show the same data; the blue curve on a linear scale and the black curve on a logarithmic scale. (Image courtesy of UWBG Grand Challenge team)

“All three of these papers represent progress on the road to more compact and higher-efficiency power converters,” Bob says. “They are also very exciting developments in semiconductor materials and device physics in their own right.”

However, he cautions that the work doesn’t mean UWBG devices are ready for the marketplace.

“There are a lot more improvements that need to be made to the transistor,” he says. “The same with the diodes. There’s a lot more optimization that needs to be done, a lot we don’t understand about their behavior.”

Researchers at Sandia and elsewhere have studied WBG materials, such as silicon carbide and GaN, for about two decades. In recent years, Sandia also has looked at next-generation UWBG materials such as AlGaN. In fact, Sandia coined the term ultrawide bandgap, which Bob says has caught on throughout the research community.

Researchers studying best way to grow new materials

One critical piece of the puzzle is figuring out the best way to grow new semiconductor materials. Researchers also must understand defects in the materials and how to process materials into working devices and find ways to improve passive elements such as magnetic inductors.

Semiconductor materials are characterized by their efficiency and effectiveness, so it’s easy to assume you could make a power supply 10 times smaller if one material is 10 times better than another. But it’s not that simple. “It depends on other components in the power converter. There’s magnetics, there’s capacitors,” Bob says. “We’re starting to look at what is a more realistic scaling.”

He and his colleagues collaborate with Sandia experts in other fields to understand the relationship of semiconductors to other components in a system. “The semiconductor enables the system, but if you have something else that’s limiting it, then you can’t reach the semiconductor’s full potential for shrinking the size of power conversion,” Bob says.

Better semiconductor materials would mean higher absolute voltages for such uses as distributing power grid energy. Right now that’s done by stacking devices in series to reach a desired combined voltage. Since UWBG materials have higher voltages than more traditional materials, far fewer devices would be needed in the stack. Bob says UWBG materials also could be useful at extreme temperatures or radiation environments — applications of interest for nuclear weapons or satellites.

Because of the potential impact on so much of Sandia’s work, Bob expects UWBG research to continue after the current project ends in September. “We lay the foundation and then we want it to continue to advance, both the science and the eventual applications.”

Authors of the Applied Physics Letters paper are Albert Baca (1766), Andrew Armstrong (1123), Andrew Allerman (1126), Erica Douglas (1744), Carlos Sanchez (1764), Michael King (1768), Michael Coltrin (1126), Torben Fortune (1766), and Kaplar. Team members also presented the results at the Device Research Conference at the University of Delaware in June and the Lester Eastman Conference on High Performance Devices at Lehigh University in August.

Sandia researchers also published a paper on the highest-ever unipolar figure-of-merit for a vertical GaN power diode grown on a GaN substrate. Figure of merit measures the trade-off between electrical conductivity and the breakdown voltage. Authors of the paper in the June 23 edition of Electronics Letters are Armstrong, Allerman, Arthur J. Fischer (1117), King, Michael van Heukelom (1768), Michael W. Moseley (413), Kaplar, Mary H. Crawford (1100), and Jeramy R. Dickerson (1123), and J.J. Wierer, formerly of Sandia. The July 21 edition of Electronics Letters published Sandia’s work in achieving the highest breakdown voltage and figure of merit ever reported for a quasi-vertical PiN diode made from AlGaN. Authors are Allerman, Armstrong, Fischer, Dickerson, Crawford, King, Moseley, Wierer, and Kaplar.

About 40 Sandia researchers work on the project in some capacity. Among those leading research focus areas are Allerman, materials growth; Armstrong, defect physics and radiation effects; Baca and Crawford, device architectures and fabrication; and Jason Neely, evaluating device performance.

Others are associated with the project, exploring potential devices and systems that could be enabled by UWBG materials. Manager Olga Spahn calls them “Friends of the Grand Challenge.” In addition, a research community outside Sandia is coalescing around UWBG — dubbed the “Ultrawide Bandgap Guild” by longtime semiconductor researcher and Sandia fellow Jerry Simmons (1000).

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.